Transmission circuits



June 24, 1930. wHlTTLE 1,767,951

.TRANSMISS ION CIRCUITS Filed Feb. 1, 1923 2 Sheets-Sheet 1 /n van for:Horace Whi/f/e by All June 24, 1930. H. WHITTLE TRANSMISSION CIRCUITSFiled Feb. 1, i925 zsheets-shee b '2 Patented June- 24, IQSO '@UNITEDSTATESP'ATEN-T OFFICE HORACE WHITTLE, OF NEW YORK, N. Y., ASSIGNOR TOWESTERN-ELECTRIC COMPANY, INCORPORATED, OF NEW- YORK, N. Y., ACORPORATION OF NEW YORK TRANSMISSION CIRCUITS Application filed February1, 1923. Serial No. 616,259.

This invention relates to transmission circuits and particularly tocoupling means for line sections having different impedancecharacteristics. s

An object of the-invention is to couple line sections of differentimpedance character istics, in such manner as to result in greateretticiency of energy transfer, lower cost, and greater simplicity of.circuit than by the methods of the prior art. A specific object is toterminate a filter-network in a combina .tion in which a filter isconnected to a line through atransformer in an improved manner. Afeature of one species of connection is the provisionof, a conductingpath for Morse or other direct currents. Another-0bi ject isto connect abalancedgmodulator With an output filter in such manner that, While theproper impedance for the transmitted currents is provided, theconnection will have low impedance for the modulating frequencycomponents impressed on the modulator.

It is well known that a inaximum'transfer of energy between twointerconnected circuits occurs when the impedance of one of the circuitsmatches the impedance of the other. lVhenever the impedances are un'equal, transition or reflection losses take place which cause aconsiderable reduction in the effective transmission therebetween. Acommon example of a means for equalizing the impedances ofinterconnected circuits is a transformer inserted between the lines, usebeing made of the well-known principle that an impedance may beeffectively transferred from the primary to the secondary of atransformer by multiplying the impedance in the primary circuit bysubstantially the square of the voltage ratio (ratio of secondary toprin'iary turns), or vice verszi. An illustra-' tion of the uscof thisprinciple is the conventional treatment of a transformer as a circuitelement, in which the transformer is simulated by an equivalent simpleimpedance. This equivalent impedance is used in the familiar Kapp vectordiagram for showingthe voltage regulation of a transformer as explained,for example, in chapter 24 of Karapetoifs Experimental ElectricalEngineering, vol. 2, second edition, 1911. In

utilizing this principle for the purpose of transformer having atransformation ratio equal to'the' square root of the impedance ratio'of thecircuits including the impedances of the transformer windings. Inan actual case, the two lines may be a cable and an open wire line andthe ratio of their impedances may vary With the frequency of the currenttransmitted. In order that a coupling arrangement of the kind describedmay beequally effective for all frequencies, the single transformer ofthe coupling arrangement explained above Would have to be replaced by aparallel arrangement of as many transformers asthere are'frequencies,each transformer having characteristics individualto a particularfrequency. Ina practical case, a compromisemay be effected by dividingthe transmitted frequency band into two or more parts by means offiltercir cuits and inserting in each part a transformer havingcharacteristics appropriate to the mean frequency of the currentstransmitted therethrough. Applicants invention, in a generic sense,

is an improvement over the arrangement described in that animprovedmethodlof relating the filters to the transformers is used. Inprior arrangements the transformer is functionally independent of thefilter and the arrangement for relating these two elements is, ingeneral, the same as that for connecting filters to other circuits. Inapplicants in-' vention the transformer combines its func: tion as suchwith that of aterminating-element of a filter, With a consequentreduction in the number of impedance elements required and therefore asubstantial reduction of the variation of the energy transmitted throughthe junction of the filter and the connected circuits atdiiferentfrequencies. The utilization of an impedanceof the transformer to supplythe impedance of a filter element is accomplished in various Ways,dependin upon the particular type of filter concerned. In the case of alow pass filter, such astmight be used for the voice frequency path, or

channel, of the coupling, the leakage induc- (this type of filter beingcharacterized by. series capacity and shunt inductance), the

, leakage inductance of the transformer is made negligible and theequivalent shunt-impedance of the transformer as measured between thepoints at which the filter is connected, whether the transformer isanauto-. transformer or a two winding transformer,

' vides a conductingjpath between-the inter I connected lines so thatthesystem may be constitutes the terminating shunt element-of thefilter. In each of these modifications, as

well as in the others to be mentioned, the ratio of the transformer'ismade to conform with the necessities occasioned by the unequalimpedances of the two interconnected circuits. a v

.In another modification of the invention similar to that firstdescribed",'the low pass filter is given a seriestermination, but inthis case the final series impedance element is not principally supplied'by the transformer. An auto-transformerhaving high impedance isconnected acrosstflie terminatingvseries element of the filteri-ifThisarrangement proused for directcurrent Morse transmission.

In a modificationof a somewhat different character, a-transformerimpedance is used as the terminating impedance of a filter inanarrangement for coupling a modulating 40 device with a'high passfilter which is designed to provide a low impedance filter for 'themodulating frequency'components, The

'equivalent'shunt impedance of the trans- ,former as measured across thesecondary constitutes the terminating shunt element of the filter whichis preferably givena full shunt I termination, the equivalent impedanceas at the primary looking inthe direction of the a different arrangementfor relating the measured across the primary, (the impedance filter)"being made great enough'to match the internal impedance of the modulatortube for th'e' frequenciesdesired 'to be transmitted.

This invention will be better understoodby' reference to the followingdetailed description taken in connection with the acc0mpany-'ing-drawing'in which Fig. 1 illustrates one coupling arrangement of theinvention embodying two distinct means'for utilizing the Y transformerinductance as the terminating" element ofafilter', Fig. 2 shows amodifica tion of the arrangement 0f ,-Fig. 1 in which transformers tothe filtersis employed; Fig.

- -3 illustrates diagrammatically the operation of the system of Figs. 1or 2 and Fig. 4 illustrates an application of the generic principle'ofthe invention to a combination of a modulator. and its outputcircuit. L

- Considering the system of Fig. l, a coupling arrangement is used toconnect a cable line 1 with an open wire line- 2. To aid manunderstanding, the invention values approximating those found inpractice will be assigned to the various quantities concerned.Accordingly, the open Wire-line may have an impedance of 600 ohms forall frequencies quencies as well as the frequencies used for voice andother methods of low frequency, signaling. 30,000 cycles may, in thisinstance, beassumed as the upper limit of the fre: quencies to beconsidered. The impedance of the cable varies with frequency, being subincluded in .the range of the usual carrier'fref stantially the same asthat of the open wire line for frequencies of approximately 135 cyclesand decreasing to approximately 140 ohms at thefrequency of 30,000cycles. The

advantage of using an individualcoupling transformer and channel foreach of the frequencies transmitted is approximated by d1- viding thetransmitted frequency band lnto two portions, and. transmitting thecurrentsof the lower frequency, characteristic of voice and telegraphicsignaling, through branch or- 1 channel 3 and the currents 'ofhigherjfrequency, through branch 4. In order to separate the frequenciesand constrain them to follow the paths indicated, thelow and high passfilters 5 and 6, respectively, areused, A transformer is used in eachbranch to equalize the limpedances of the interconnected lines for themean frequency of the currents transw mittedtherethrough.

Equality is indicated when the impe'dances 7 looking in either directionfrom the terminals or, as would naturally follow from the previousadjustment, when the impedances,1ooking in either direction from theterminals of the secondary, are equal. The equallzation in either caseequal to the square root-of the ratio of the interconnected impedancesfor the mean frequency of the currents t'ransmitted therethrough. Properselection of the" transformer ratio accordingly insures the avoidanpe ofreflection and" transition losses at the-junction comprised by thetransformer.

The problem remains of terminating the filters and connecting them tothe open Wire line and to the transformers so thatlosses are avoided atthese junction points. q

.Consideringthis problem generally, without reference to a particularbranch ofthe' present coupling arrangement or toy a particular typeof'filter, except-that some modification of the usual iterative orperiodic filter circuit illustrated by the Campbell Patents No.1,227,113 and No. 1,227,114, issued May 22, 1917, will be assumed, itmay be said that 10F of the primary of the transformer are equal 11b iseffected by making the transformerratio ance. For purposes of sectiondesign such a filter is conveniently assumed to have infinite extensionin either direction. In such an 1nfinite filter the impedance, lookingin the direct'ion of the infinite extension, is the same for allcorresponding points of the filter. This impedance is known as thecharacteristic impedance of the filter measured at that type of crosssection. For practical purposes, of course, a filter must be givenfinite dimensions. In order to provide for the combination of a finitefilter witha load circuit connected to either end without transition orreflection losses such a combination must be made to be the equivalentof an infinite filter, the loadcircuit, accordingly, .replacing theinfinitely extending portion of the filter. Since the load impedance isfixed this requires the proper choice of filter. If the filter isproperly designed relatively to the connected load it may have a fullshunt or a full series at either end, these terms denoting,respectively, that a whole shunt section element or a whole seriessection element is used to terminate the filter. If the filter isterminated in the middle of a series element the termination ismid-series, if a terminating shunt im-v pedance has a magnitude twicethat of the regular shunt impedance, the termination is mid-shunt. f I

The design of the filter terminations in the system of the invention isgoverned substantially by the considerations above,.except as limited bythe fact that, according to the essential novel principle of theinvention, a transformer of proper ratio is used and the impedance ofthe transformer constitutes the terminating impedance element of thefilter.

Considering branch 3 of Fig. 1 indetail, the filter 5 is of theconventional low pass type and may have many or few sections. Itsconnection with line 2 may be governed by the considerations aboveexplained. An impedance of the transformer must constitute theterminating impedance of the low pass filter at that end, and anytransformer impedance,

from the inherent nature of the device, must be substantially inductive.Since the inductances of a low pass filter are theseries elements, thefilter termination must be of the series element type. I

In order to enable the transformer to sup ply this impedance it isdesignedjto have a large leakage inductance. A leakage inductance may,according to well known theory, besimulated by an equivalent inductancein series with the transformer windings, as shown by 9 and 10.Accordingly, by proper design ..of the transformer, the serlesterminating inductance of the filter may be supplied by the equivalentleakage inductance of-the transat the left, instead of at the right ofthe transformer 7, or it. may be duplicated at'the left. In other words,the transformer may be e1- fectively used at either end or at any partof a filter intermediate the ends. If used inter- -mediate the ends thedesign of thetwo parts of the filter must, of course, be different tocorrespond with the impedances. of the respective lines to which theyare connected. In either case the ratio ofthe transformer is made suchas to match'the impedances of the two circuit combinations; The completeequivalent circuit of the transformer comprises, in addition to the loadand the series-leakage impedances one of which is by the above indicatedmethod. of calculation transferred to the filter side, a shunt circuitconnected between the two leakage impedances and carrying themagnetizing component of the primary current. The impedance of thisshunt circuit is found to be, in accordance with the fundamental. theoryof transformer operation the mutual-inductance of the transformermultiplied by the ratio of the primary to secondary turns. For atransformer having zero leakage inductance this impedance is'simplyequal to the self inductance of the particular winding at whoseterminals the calculations are chosen to be made. In the present caseatransformer is used in which this equivalentshunt impedance is great as,compared with the normal shunt impedance of a filter section andaccordingly does notdisturb the circuit relations in thefilter-connection.

Considering branch4, a shunt termination at, thefilter end is required,since a high pass filter is used, in which type of filter theinductanc'e elements occur in shunt. In the case ofbranch 4, as well asof branch 3, a full or a partial shunt termination may be used, theparticular type of shunt termination affecting only thei-mpedance of thefilterline combination and accordingly the-ratio of transformation. Theration of transformation is fixed. by the impedance ratio of theinterconnected circuits, one or both of which consist of such afilter-line combination, as

explained. The equivalent shunt impedance of the transformer determinedas described in the paragraph above should be made that suitable for theterminating inductance of a shunt-terminated filter. Since'theintroduction of a series inductance into a high pass filter woulddisturb its normal function, the transformer should have negligibleleakage and accordingly an auto-transformer 8, as

shown, is preferred. For this case the equivalent shunt impedance wouldequal the self- {inductance of the transformer across the pair and thetransformer. ,branches 3 and 4 are designed as'lndicated thecircuits maybe used equally well for transmission in either direction.

' The use of the coupling transformer to constitute-an element ofthefilt'er'results in a marked economy of impedance elements, as

' compared with the conventional arrangements in which the transformerand filter are functionally independent. For example, the use of'thearrangement in branch 3 results in the saving ofone coil, that in branch4 inthesaving of one coil or two coils, depending upon whether anauto-transformer or a twowinding transformer is replaced by thisarrangement. Q Although the arrangements in the branch circuits of-Fig-lingeneral conform, as to the choice o'f' elements, with most" efiicientpractice, they may be varied incertain details, as shown for example inFig. 2, in which similarly numbered elements have similar'functions. Inthis figure the circuits of. branch 4- differ from thecorrespondingcircuits. of'Fig.' 1 principally in the use 'of a two-windingtransformer instead of an autotransformer. Since the transformer is ofthe two-winding type special precautions should be taken to insure thatthere is negligible leakage.-- The circuit illustrated may be used,depending on the values of the capacities, either as a pair ofconnectedfilters or as a single filter. If the latter the condensers on oneside'of the transformer are used only as blocking condensers. Branch 30fFig. 2 differs essentially from that of Fig. 1 in that reliance is nothad'on thelea'kage inductance of the transformer to supply the seriesterminating inductance of the filter since an auto-transformer, whichhas no leakage inductance, is used. Elements 9 accordingly areinductances constituting the con- -ventional terminating elements of thefilter.

Although the use of an auto transformer, as compared with a two-windingtransformer, is

attended by a lesser economy'of circuit lements, since it cannot supplythe series terminatmg inductance of the filter,-it has the compensatingadvantagethat by its use a condueti ve path is provided through thebranch whlch, accordingly, permits of the coupling to be used for directcurrentMorse, as well as carrier and ordinary telephone communication, afeature not present in the system of Fig. 1. The condenser 12functionsas a blocking condenser to prevent a direct cur-- rent shortcircuit. 'Since a series filter termi nation used, the equivalentslluntingimpedance of the transformer, including the condenser12,as'measurcd at the terminating points of the filter (in this particularcase it branch 3 of Fig. 1. and for the same reason.

Series inductances are shown'oneach side of the line insteadof thesimpler arrangement of Fig. 1.'

Fig. 3 shows,'for an actual case, the resultant effect in the system asa whole of a coupling arrangement of Figs. lor 2. If the filters inbranches 3 and 4 are designed to attenuate substantially all currentsexcept those in the voice and carrier frequency bands, re- 'spectively,graphs A and B represent the currents passed by these branches. Curve Cshows the summation in thelines' 1 and 2 of the currents in thebranches.

v of the ordinates of curves A and B, this is not true for a smallportion of the range of each filter at the cut-off point. This resultsfrom a pronounced phase displacement between the component currents atthis point, although there is a progressive change in phase withfrequency and a similar but smaller interference effect at other points.It is to be noted that this results in a sharper cut-off in theresultant curve than in either of the component curves. If the cut-offfrequencies are brought more closely together, so that curves A and Bare closer together, the dip in curve C can be substantially eliminatedsothat its ordinates are substantially uniform. I

Fig. 4 shows .an application of a principle, quite similar to thatillustrated in branches 4 of Figs. 1 and 2, in a combination comprisinga balanced modulator working into a high pass filter having asufliciently high frequency lower cut-off to eliminate the modu; latingcomponent. In the arrangement shown there is provided, as will beexplained later, a modulating. system in whichthe unmodulated carriercomponent is suppressed and in which the output circuit may be given alow-- impedance for both of the components. im-

1,672,056, issued June 5, 1928, this condition results in greatlyincreased-elficiency.

The balanced modulator disclosedin the drawing is of the conventionaltype as dis; closed, for example, in a paper by Colpitts and Blackwell,Carrier current telephony and telegraphy, volume 40, No. 4- of theJournal of the A. I. E. E., page 314. By the I i arrangement shown,modulation of the carrier current from source 23 by current from voiceor any other low frequency circuit It is noted that circuit. Thesuppression of this carrier fre-- 14 is attended by a suppression of theun-. modulated carrier-component in the output quency component-resultsfrom the opposed effects of equal potentials induced in the secondarywinding 16 of the three winding transformer 17 by the amplified highfrequency currents from source 23 inprimary 15. The opposition ofpotentials in primary 15 results from the mode of impressing potentialson the input circuits 'ofthe tubes from source 23, these potentialsbeing equal and having the same phase. Currents of the modulatingfrequency from circuit 14 are not suppressed since potentials impressedfrom this circuit on the input circuits'of the two tubes are of oppositephase, andaccordingly the resultant potentials in the output circuits ofthe tubes, and hence in the secondary of trans- "formers 17, are notopposed as in the case of the similar potentials derived from source 23.The currents of the side band frequencies partake of the character ofthe modulating currents, which constitute one of its two combinationelements, and are accordingly not suppressed. This result is easilydeduced from a consideration "of the physics of the operation of thebalanced modulator. The currents from circuit 14, by unsymmetricalsuper-position with the currentsfrom source 23 on the input circuitsofthe tubes, destroy the bal anccestablished by the current from that.

source alone. p

The transformer and 1tsassoc1ated filter 18 is designed 'so that theequivalent shunt impedance has such a value asto constitute the.

transformer the shunt: termination of the filter 18, and so that theimpedance ofthe output circuit including the transformer and filterlooking away from the modulator has the value, in proportion to theinternal impedances of the tubes, which is demanded for efiicienttransmission of the selected components.

Since the inductances in a high pass filter are in the shuntrather thanin the series,

elements the transformer should have as littleleakage as possible. Theuse of an auto transformer, as in branch 4 of Kg. 1, would insure zeroleakage and this type of transformer could be used instead of the two-'winding transformer illustrated. Consistent with the requirement thatthe equivalent shunt impedance must supply the. shunt terminatingimpedance ofthe filter, that transformer, in a przi'ctical case,'. wouldbe designed to have a much smaller equivalent shunt impedance thru themore conventional designs of transfo. ier that would otherwise be usedto couple the filter with the modulator. .Withthe usual values of tubeand line impedances, a relatively large ratio of primary to secondaryturns would also be Since the. shunt terminating impedance of a highpass filter of the type illustrated is low for the usual modulatingfrequencies, there is provided a modulating system in which the outputcircuit-has'a low impedance for the modulating current. with consequentincreased efiiciency of the system and without sacrifice ofthe'etficiency which normally results from the proper relation betweenexternal and internal output impedance for the' frequencies of the currents transmitted and with a marked edonomy of circuit elements. Thetype of filter disclosed in Figs. 1 and required in order to effect theproperlim- 'pedance matching.

2- is denominated the two-element type,

since as above explained each section consists of two elements, namely,one series element and one shu'nt element. The particular design offilter disclosed in Fig. 4 is variedfrom this type by the use of aresonant circuit 19 for the shunt circuit of one section. A filterhaving its, sections made up of this type of shunt circuit (having twoelements) and a single-series element is known as a threeelementtype.-Italso belongs to the class of filters shown and described in U. S.

patent to Campbell 1,493,600, issued May 13, 1924 known as thesuppression type-l, which are'characterized by the use of eitherresonant shunt circuits, resonant loops in the line orby the use of boththese features. An

advantage in the use of the three element suppression type of filterresides in the sharper cut-off that can be secured thereby as comparedwith the two element type.

Since the terminating impedance at. the

modulator end is eifectively an inductance and, therefore, theterminating shunt impedance of a two-element filter'sectiomthefilter, asa whole, can be considered as of I the compositetype having twopart-sections one part-section of a conventional two-ele' ment typefilter and the other part-section of the three-element suppression type,the condenser-s20 and 21 each effectively comprising a seriesterminatingcapacity of each part.- section. It is preferable to use a full shun-ttermination for this filter 'at the modulator end, in order to have thelowest possible im- 1. In combination, an input circuit, atransformer, abroad-band filter and a load circuit in se ries relation, the filter andload circuit having characteristics substantially equivalent, to that ofan infinite filter having identical sections, and an impedance of thetransformer constituting the shunt terminating' impedance of the filter.2. In combination, .two circuits of different impedance characteristicsand a coupling means therebetween, said means comprismg a non-unltyratio'transformer and a matching the impedance of its connected cir-o"cuit, the impedance of the transformer, filter,

broadband filter, the impedance of the filter and connectedcircuitmatching the impedance of the other circuit, and an impedance ofthe transformer constituting the terminating impedance of the filterconnected thereto;

3. A combined filter and a non-unity ratio transformer, an inductance ofthe transformer in effect forming part of the filter and the filterconsisting of a line of negligible attenquencies lying outside of saidlimiting fre uation containing lumped impedances effectively in serieswith the line, and lumped impedances effectively in shunt across theline, said impedances having precomputed values dependent upon'the upperlimiting frequency and the lower limiting frequency of a range offrequencies it is desired to transmit with-o out attenuation-the valuesof said series and said shunt impedanees being so proportioned that thefilter transmits with practically negligible attenuation sinusoidalcurrent of all frequencies between said two limiting frequencies whileattenuating, and approximately extinguishing, current of neighboringfrequencies.

4.- In combination, a non-unity ratio stepdoWn transformer, a high passfilter, a load circuit connected to the filter at'the end remote fromthe transformer and a circuit connected in energy transfer relation tothe transformer, thelequivalent shunt inductance as measured atthe lowwinding of the transformer constituting the shunt terminating inductanceof the filter,"saidinductance being low for frequencies below thecut-off frequency, and the impedance of the transformer filter, and loadcircuit substantially matchhigh frequency wave thereon, and a combinednon-unity ratio transformer and multi'-sec-- tion filter connected'tothe output circuit of said modulator, the. equivalent shunt inductanceof the transformer as measured-at the points at Which itis connected tothe filter in effect forming part of one section of-the filter' and thefilter having its cut-off sharpened by the replacement of asectionthereof by a section ofdiiferent type whereby its atten-' uationcharacteristic is made steeper near its critical frequency;

7. In combination, a modulator, means for impressing a relatively lowand a relatively high frequency Wave thereon, and a combined non-unityratio transformer and m'ulti-section filter connected to the outputcircuit of said modulator, the equivalent shunt inductance of thetransformer in effect forming, part of one section of the filter and onesec tionof the filter-givinghighattenuation over one frequency range andanother section over another frequency range, whereby the multisectionfilter gives high'attenuation over both ranges. j

8. In combination, a modulator, means for impressing a relatively lowand a relatively high frequency Wave thereon, anda combinednon-unitytransformer and filter connected to the output circuit of saidmodulator, the filter I comprising two types of sections having acommonfinite critical frequency, an inductance of the transformer in effectforming.

part of one of said sectionsand one type of section giving highattenuation at a frequency remote from the'critical frequency andanother type of section giving'high at-. ten-nation at a point near thecritical frequency. a r

9. In combination, a modulator, means for impressing a relatively lowand a relatively high frequency wave thereon, and acombined non-unityratio transformer and 'multi-section filter connected to the outputcircuit. of' said modulator, an inductance of the transformer in effectforming part of one section of the filter, one section of the filterhaving another section having maximum attenuation .at a frequency nearthe cut-off frequencyin vter connected to the output circuit of. said 1modulator, one section of the filter having. in effect a series armcompris ng lumped capacity and a shunt arm comprising the in- 'maximumattenuation at zero frequency, and

ductance of the winding of the transformer connected thereto, andanother section of the filter having, in effect, a series arm.comprising lumped capacity and a shunt arm comprising lumped capacityand lumped inductance being low for said transformer, filter,

pass filter connected thereto for passing the modulated components andattenuating the modulating components, and "a load circuit connected tothe remote end of the filter,the filter having in effect seriesimpedance comprising lumped capacity elements and a shunt terminatingimpedance comprising the equivalent shunt inductance of'the transformeras measured at the points at which the filter is connected, saidequivalent, shunt inducthe frequencies below the cut-off frequency andthe impedance of and load circuit substantially matching the impedanceof the modulator for the frequencies to be transmitted. v

- 12. In combination, a modulator including an output transformer, ahigh pass filter for passing the modulated components and a loadcircuit, the ratio of the transformer being such that the'impedance ofthe transformer, filter, and load circuit, is substantially equal to theinternal impedance of the modulator at the frequencies of the currentstransmitted therethrough, ing two types of sections having the samefinite critical frequency,

' at which the filter is connected in effect formthe transformer,

' tion 7 v ing the filter with the ing part of one section giving highcy remote another nation at a point near 13. In combination,

attenuation-at a frequenthe critical frequency. a balanced modulator Iincluding an output transformer, a load cirquit, and modulatedfrequencies coupling sa1d f lator and load circuit, said full shunttermination constituted by the a high pass filter for passing themodufilter having a equivalent shunt inductance of the transformer asmeasured at the'points at which the filter is connected, andtheimpedance of filter and load circuit substantially matching theimpedance of said modulator at the frequencies of the currenttransmitted therethrough.

14:. In combination, a modulator, means for impressing a relatively lowand relatively high frequency wave thereon, a multi-sechigh pass filter,

output circuit of said modulator, said filter of the'two-element typewith a full shunt termination and another section of the threeelementsuppression type, and a load circuit connected to the end of the-filterremotefrom said two-element section,' tlie' equivalent shunt impedanceofthe transformer asmeasured at the points at which the filter isconnected constituting the shunt terminating impedance of the filter andthe transformer, filter and the filter compristhe equivalent shuntinductance of the transformer at the points of said sections, one typeof from the critical frequency andtype of section giving a high attenatransformer connect-. I

having an end section load circuit substantially matching the pedance ofthe modulator for the frequencies In witness whereof, I hereuntosubscribe my name this 24th day of January,

ome wHrrTLn.

